Rapoport and co-workers have shown the potential of 9, 10-[3H] palmitate for imaging brain tumors in rats. The agent has proved to be a sensitive probe of altered brain lipid metabolism during development, aging, and after physiologic and pharmacologic manipulation. dl-Erythro-9,10-[18 F]difluoropalmitate (DFPA) has been synthesized to facilitate the clinical application of lipid probes for brain imaging by positron emission tomography (PET). An objective of this project was to prepare [18 F] DFPA as a potential analogue of 9,10-[3 H] palmitate for brain tumor imaging in rats. Brain tumor imaging by [18 F] DFPA was carried out in male Fischer-344 rats which were intracerebrally implanted with Walker 256 carcinosarcoma tumor cells. Rats were killed after 20 min. Brains were removed and either prepared for autoradiography, or brain and tumor were separated and their radioactivity quantified by scintillation counting. Brain tumors were well demarcated from surrounding and normal brain in autoradiographs and closely paralleled tumor growth in histological sections. Small intracerebral tumors (< 0.5 mm diameter) possessed the highest incorporation of [18 F] DFPA, 4-fold greater than normal brain, and could readily be differentiated in autoradiographs. Studies are continuing to facilitate the clinical application of [18 F] DFPA in brain tumor patients. A paper dealing with the synthesis and biodistribution of [18 F] DFPA in rats is in press (Int. J. Appl. Radiat. Isotopes, Part 1,) . An abstract dealing with the biochemistry of this compound in normal brain and brain tumors has been reported (Abst. Soc. Neurosci. 15: 857, 1989).